where we left off
6 out of 8 recovered tags stopped recording 13-38 days after tagging, i.e. the animal died.
| tag_serial_number | release_date_time | detected | sex | life_stage | length1 | length1_unit | weight | weight_unit | capture_depth | recapture_date_time | animal_id | tag_type | scientific_name | release_latitude | release_longitude |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1293322 | 2018-07-19 09:00:00 | no | f | unknown | 80 | cm | NA | NA | 3-20m | 2019-09-29 | 3175 | acoustic-archival | Mustelus asterias | 51.61002 | 3.649600 |
| 1293319 | 2018-07-19 12:00:00 | yes | f | unknown | 73 | cm | NA | NA | 3-20m | 2018-08-21 | 3178 | acoustic-archival | Mustelus asterias | 51.61177 | 3.649833 |
| 1293295 | 2018-07-19 13:00:00 | yes | m | adult | 78 | cm | NA | NA | 3-20m | 2018-09-27 | 3182 | acoustic-archival | Mustelus asterias | 51.61250 | 3.653783 |
| 1293304 | 2019-07-11 10:25:00 | no | f | mature | 72 | cm | 1.35 | kg | NA | NA | 5067 | acoustic-archival | Mustelus asterias | 51.61502 | 3.657802 |
| 1293310 | 2019-07-11 12:20:00 | no | m | mature | 72 | cm | 1.40 | kg | NA | 2019-09-03 | 5069 | acoustic-archival | Mustelus asterias | 51.61822 | 3.663183 |
| 1293312 | 2019-07-12 08:15:00 | no | f | unknown | 91 | cm | 2.80 | kg | NA | 2019-08-29 | 5218 | acoustic-archival | Mustelus asterias | 51.61058 | 3.656550 |
Looking at the 6 tracks in more detail, it strikes that 3 sharks swam to the wind farms while 3 sharks stayed very close to the release location, namely tag 304 (female, 13 days of liberty), tag 310 (male, 37 days of liberty) and tag 312 (females, 32 days of liberty). Those 3 sharks were released in 2019.
It’s hard to say if they followed the submarine cables to the OWFs…
depth and temperature profiles
This shark (male) swam out to the OWF. Comment on ETN: Washed ashore around 27/09/2018 Found at Westenschouwen, tussen domeinen twee en t oude Vuurpad.
This shark (female) stayed in the Oosterscheldt.
This shark (male) stayed in the Oosterscheldt.
This shark (female) stayed in the Oosterscheldt.
This shark (female) swam out to the OWF. Comment: found on 51°49’32.4”N 3°53’30.4”E (lat 51.825667, lon 3.891778)
This shark (female) swam out to the OWF.
All of these 6 sharks drift at 20-35 m depth (they are dead?) before surfacing. Can you differentiate any fates here or do you think that would be possible?
depth and temp profiles of long term DST data
These are the profiles of the 2 tags that recorded for over a year.
This shark (female) swam south into the English Channel.
Between October 2018 and April 2019, this female dove deeper than the tag’s pressure sensor limit of ~75m. What could this mean?
This shark (male) swam north into the Southern North Sea.
The male stayed at the surface much more compared to the female. What might he have done?
what I plan to do now
I would like to segment the vertical data logs into behavioural patterns. This was done by Heerah et al., 2017 (https://doi.org/10.1186/s40462-017-0111-3). I spent a day with it already, but it is not trivial to format the code to the M. asterias DST data. To correctly do this, I would need the help of someone that has already done this. Jolien supervised a thesis in 2020 (Camille Boileau-Locas), that did that as well, but I know she has a lot to do right now. Mathieu Woillez also co-authored the paper by Karine Heerah, so I could contact him as well, potentially. What do you suggest? The method of Heerah et al., 2017 uses STFT, a modified version of FFT. I was thinking that I maybe at first do a regular FFT to look at the periodic patterns of the signals.
I would like to assess the fate of the 6 sharks that only logged for < 1 month. Currently, I have in mind that I could try to find rules and translate that into a fate (e.g. temp rise by xy degrees + regular surfacing intervals = fate_seal). Do you think I have any chances of trying to identify different fates(if there are any)?
Griffiths et al., 2020 (https://doi.org/10.1371/journal.pone.0239480) did not use any fancy/complex Hidden Markov Models like Heerah but they calculated statistics such as the time spent at certain depth bins, vertical speed, seabed proximity, or vertical movement behaviour (based on a Wilcoxon test). This I would perform as a backup/complementary analysis with secondary priority, because I find the behavioural segmentation more interesting.
Do you have any other analyses in mind that could shed light on behavioural patterns of the sharks?